90 Park Ave with a serious ABS problem

I am a retired auto mechanic and I have a 1990, good looking very good running Park Ave. It recently developed an anti-lock brake problem. For awhile the anti-lock and brake warning lights were coming on ocassionally, when the brake pedal was depressed. I checked out all connections, fluid level, checked the calipers, hoses, lines, wheel cylinders, etc. and found no problems.
The other day, the problem got worse. Now both the anti-lock and brake warning lights are on constantly...the pedal is hard.. and the brake fluid reservoir is literally overfilling and spilling out, on its own. The car is obviously not drivable, in this condition.
To further complicate the problem, the car is a 1990, which used the hydraulic pump assembly type anti-lock system, instead of the more familiar ...booster/master cylinder & anti-lock system, which Buick switched to in 1991. The hydraulic pump assembly or any of its parts are not even listed in any aftermarket parts catalogs. However, the parts are still available from a Buick dealer. But, the hydraulic"master cyclinder" portion costs $1650,00. The pump motor costs $295.00. So, basically its about a $2,000.00 set up. Which is what I paid for the whole car. So, used parts are the only way to go for me.....
I was hoping that my description of the problem, would "ring a bell" with someone who has seen this problem before. Because unfortunately, I have not. And most of the ABS problems that I encountered I used to send them off to a dealer, for service. This is not a financially an option for me..
I would greatly appreciate a little help, in pointing this old mechanic in the right direction!

i understand exactly how you feel,im going through the same problem with my 1989 buick parkave with abs setup.i changed the brake hoses and calibers and the brakes seem to be locked up in front.i bleeded the brakes and still the anti lock light and brake warning light is still on.this is enough i went and got the old master cylinder and brake booster and im switching over because i have alot of money invested just did motor and tranny job now this is in the way!my suggestion switch to regular booster or buy another car without it.

I think johnnyguitar has the right idea. If your fluid overflows, you're probably pumping air into (everything) and need those parts. Get the non-ABS parts, and remove the bulb in the dash. My 95 is just fine without it...rain, snow or dry. How many years did you drive without ABS?